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Chapter 7: Exposure and Signal
This chapter discusses the various factors that affect the signal acquired on an array,
including array architecture, exposure time, temperature, and saturation.
7.1 Array Architecture
A photodiode array (PDA) is a two-dimensional, rectangular array of photodiodes attached
to an integrated circuit (IC) chip. For spectroscopy, a PDA is placed at the image plane of a
spectrograph to allow a range of wavelengths to be detected simultaneously.
Each sensor on a PDA is connected to its own charge storage
well
. These sensors respond to
the intensity of light or infrared (IR) radiation falling on their collection areas and store a
proportional amount of charge in their associated wells. Once charge accumulates for the
specified exposure time that has been configued within the software, the charge in each well
is read out serially.
7.2 Exposure Time
Exposure, which is configured on the
Experiment Setup|Main
tab {
Common Acquisition
Settings
expander}, is the time between start and stop acquisition commands sent by the
application software to the detector. In combination with triggers, these commands control
when continuous cleaning of the array stops and when the accumulated signal will be
readout. The continuous cleaning prevents buildup of dark current and unwanted signal
prior to the event of interest. At the end of the exposure time, the array is read out and
cleaning starts again.
NOTE:
The NIRvana-LN camera has a minimum exposure time of:
• 100 μ s in 250 kHz mode;
• 200 μ s in 100 kHz mode.
7.2.1 Exposure with External Shutter
When a camera does not have an internal shutter, either the light source must be controlled
or an external shutter must be used to block light from the array during readout. If the light
source can be electronically controlled using the
SHUTTER MON output, the FPA can be
read out in darkness.